1. Field of the Invention
The present invention relates to a robot controller having a control system mode switching function, and in particular to a robot controller which can smoothly switch the mode between a position control in a free space and a position or force control to a contact surface, so that even if the contact surface has an unknown geometric error, the control system mode can be switched without the damage of a contact object and a workpiece and the out-of-control of the robot.
2. Description of the Related Art
As for force control study cases, there are known a hybrid position/force control (M. H. Raibert, John J. Craig: Hybrid Position/Force Control of Manipulators, Journal of Dynamic Systems, Measurement, and Control 102, ASME, pp. 126-133, 1981, hereafter referred to as Document 1), a method of developing it using operating coordinate systems as references (O. Khatib: A Unified Approach for Motion and Force Control of Robot Manipulators: The operational Space Formulation, IEEE Journal of Robotics and Automation, Vol. RA-3, No. 1, 1987, hereafter referred to as Document 2), a mechanical impedance control (N. Hogan: Impedance Control: An Approach to Manipulation: Part 1-3, Journal of Dynamic Systems, Measurement, and Control 107, ASME, 1985, hereafter referred to as Document 3), and so on.
Each of the cases is a pioneer study regarding the robot force control. It is, however, known that those cases may encounter a problem in that a control characteristic extremely deviates from its design specifications or a control system becomes unstable due to variations in elasticity and frictional coefficient of a contact surface defining a restricted space.
Further, the most of the cases are limited to the force control in a normal direction to the contact surface.
Moreover, no sufficient consideration is given to a control system with respect to an uncertain state transition associated with the collision occurring between the free space and the restricted space.
As for a control method for collision processes (Shoji, Inaba, Fukuda, and Hosogai, Stable Control for Robot Manipulator facing collision, Japan Mechanical Association Symposium (Chapter C), Vol. 56-527, pp. 1847-1853, 1990 or the like, hereafter referred to as Document 4), there are known a method in which an optimum approaching speed prior to the contact is preliminarily obtained so as not to cause the large impact force, and a position control is switched over to a force control at the time point when a detected force value succeeding to the contact exceeds a threshold value (Kitagaki and Uchiyama, Optimum Approaching Speed for Manipulator in Ambient Condition, Japan Robot Association Journal Vol. 8-4, pp. 413-420, 1990, hereafter referred to as Document 5, Shimura and Hori, Robust Force Control for Robot Manipulator and Control for Collision Process, Japan Robot Association Journal vol. 11-2, pp. 235-245, 1993, hereafter referred to as Document 6; and Kiyoshi Oishi, Force Control Containing Collision Process Based on H .infin. Speed Controller, Japan Mechanical Association Robo-Mech 95, pp. 358.361, 1995, hereafter referred to as Document 7), a method in which a control mode having a large damping characteristic is once inserted at the time of switching (O. Khatib, J. Burdic, Motion and Force Control of Robot Manipulators, IEEE Conference on Robotics and Automation, pp. 1381-1386, 1986, hereafter referred to as Document 8), and so on.
In those controls, only the depressing force in the normal direction is used as an object to be force-controlled, and therefore, the frictional force in a tangential direction can not be dealt with, or otherwise the switching therebetween can not be carried out as desired.
It is essential to the force control study cases (Documents 1, 2 and 3) that the geometric position of the contact surface is completely known, that is, no consideration is given to a contact surface having an uncertain geometric error in these cases. For this reason, if the contact surface has the geometric error, the position control under the contact state may result in unnatural force acting on and damaging the contact object and the workpiece, or otherwise, the force control under the non-contact state may result in the out-of-control of the robot because of attempting to make the depressing force coincident with a target force value.
Further, each of the study cases (Documents 4, 5, 6, 7 and 8) is directed to a control system which is simple and lacks a robust property, or is based on a control system which may be robust but is not reliable in stability as a whole.